New hepatitis agents continue to be recognized. Recently, a form of epidemic hepatitis occurring in India was found not to be caused by any of the recognized hepatitis viruses. Previous attempts to transmit an agent from acute-phase clinical samples to primates were partially successful. Transient low-level liver enzyme elevations and histopathologic changes consistent with hepatitis in liver biopsies were observed in some animals, but this was not uniform, and attempts to serially transmit an agent in chimpanzees and marmosets also produced irregular results. Efforts to identify viral agent(s) in recently obtained clinical specimens from India, Algeria and the USSR include primate transmission studies, immune electron microscopy, and cloned cDNA hybridization. Although an etiologi agent has not been identified in experimental animals and none of the specimens has hybridized with probes that detect a wide range of picornaviruses including HAV, several different virus-like particles have been visualized. Attempts to define a serologic relationship between hepatitis and one or more of these particles are in progress.